The utilisation of space provides many opportunities to deliver pioneering innovations during the 21st century. One of these opportunities is the gossamer spacecraft, an emerging technology to achieve very low mass, large area and low stowage volume. Examples include large ultra-lightweight membrane reflectors and distributed tethered formations. Gossamer spacecraft offer the potential to deliver innovative new science and applications missions to aid our growing globalised societies: high-performing communications antennae, scientific telescopes and space-based solar power collectors. However, the ability to control such large structures in space is essential for their successful operation. To this aim, this thesis investigates a novel means to control large gossamer spacecraft by exploiting modulated solar radiation pressure (SRP), thus by modifying the nominal light pressure acting on the structure in space. Various concepts have been proposed in the past to control the attitude of a gossamer spacecraft, employing complex mechanical systems or thrusters. Furthermore, methods to control the surface shape of a large membrane reflector using, for example, piezoelectric actuators, are being developed. Since on-board control systems need to be high-performance, reliable and importantly lightweight, this thesis investigates the use of thin-film reflectivity control devices across the spacecraft surface. Controlling the reflectivity modulates the Sun's light pressure acting on a thin membrane thus controlling its shape. In addition, body forces and torques become available to control the attitude of such a large structure 'optically', without using traditional mechanical systems. The concept is demonstrated first by controlling a two-mass tethered formation in a Sun-centred orbit, showing that the spacecraft attitude can be stabilised around new equilibria created by controlling the surface reflectivity of the masses.Subsequently, the concept is applied to control the attitude of a large membrane reflector, which confirms the viability of reflectivity modulation by generating variable optical torques in the membrane plane. In particular, the nominal SRP forces are modified by introducing different surface reflectivity distributions across the membrane. It is shown that through these optical torques, the reflector can be steered, for example, to a Sunpointing attitude from an arbitrary initial displacement. The analysis also considers the variation of the SRP force magnitude with changing light incidence angle towards the Sun during the manoeuvre, thereby presenting solutions to a challenging attitude control problem. Furthermore, by adopting a highly-integrated multi-functional design approach, the concept of reflectivity modulation is also employed to control the surface shape of a large membrane reflector. First, the nominal (non-parabolic) deflection shapes due to uniform SRP across the surface are presented. Subsequently, a closed-form solut ion for the reflectivity function across the membrane required to create a true parabolic deflection shape is derived. In order to improve the quite large focal lengths of the deflected shapes that can be generated for a tensioned membrane, shape control of a slack suspended surface is also considered. The achievable (shorter) focal lengths support the feasibility of exploiting modulated SRP for controlled surface deflection. In summary, this thesis demonstrates the potential of using surface reflectivity modulation to control the attitude and morphology of large gossamer spacecraft without using complex mechanical systems or thrusters. Therefore, the concept of optical control represents a major step towards highly-integrated adaptive gossamer structures and supports the development of this promising key-technology to deliver advanced space applications.
|Date of Award||7 Oct 2015|
- University Of Strathclyde
|Supervisor||Colin McInnes (Supervisor) & James Biggs (Supervisor)|